Rolling contact measuring apparatus utilizing a magnetic counting device



Mardi 5, 1957 T. M. BERRY 2,783,540

ROLLINGA CONTACT MEASURI'NG APPARATUS UTILIZING A MAGNETIC COUNTINGDEVICE Filed Sept. 50. 1953 2 Sheets-Sheet 1 s a f Puff fz/sf Haw/WW@ Fil. f/a/g- AHH/mm. cva/Vrin.

March 5, 1957 T. M. BERRY 2,783,540

ROLLING coN'rAcT MEASURING APPARATUS unuzmc;

A MAGNETIC COUNTING DEVICE A Filed Sept. 30. 1953 2 Sheets-Sheet 2Attorney.

iinited States Patent O ROLLING CoNrAcr MnnsURiNG APPARATUS UTILiZrNG AMAGNEHC coUN'riNG DEVICE Theodore M. Berry, deceased, late ofSchenectady, N. Y.,

by Albert L. Berry, administrator, Jewett, Kans., assignor to GeneralElectric Company, a corporation of New York Appiication September 30,1953, Serial No. 383,212

1i? Claims. (Cl. 331-134) ber oi' revolutions of the wheel may then beused as Va',

measure of the length of the material Whichhas passed by it.Occasionally, the rotating wheel directly drives' a counter, but moreoften it is required remotely to indicate the number of revolutionsmade. *Inl this case, a

commutator, cam, or photoelectric systemmay beattached to the rotatingwheel in order to furnish impulses for a suitable counter.

Counting devices or" this type are subject to 'wearand" misoperation`due to dirt and foreign matter being deposited in the commutator, and,in addition', the commu. tator adds friction to the measuring device andmay cause slippage between the rotating wheel and the movingfmaterialbeing measured. Such devices may also be opera tionaily limited in thespeed at which theywill operate-i" Thus, if the rotating wheel isreduced in diameter. in orderA to have a more accurate measurement,thereby increasing the speed of rotation of the wheel, it'may become`necessary to decrease the speed withwhichthe material moves past thewheel, in order to comply withthespeed limitation inherent inthedevice.l

Therefore, it is an objectiofjthe present' invention to provide-acounting device which may be adapted to count revolutions or movementsof an element, andY whic'h'is frictionless, is operable at anyreasonable speed, arid'is sealed against dirt and other contamination.'v

One form of the present invention wherein "the foregoing objects areattained comprises an electronic discharge device, of thetype whereinthe ilow of current may be varied by applying a magnetic field to thedevice, and a permanent magnet, which is` secured to the 'element Themagnet and4 electronic discharge device are soY positioned relativeVwhose motions are' to, be counted.`

to each otherthat, asthe magnetV moves, its'm'agnetic eld passes throughthe dischargedevice atleast once during every 'cyclevof motion. Meansare 'lprovidedi to count the variations inthe dow of` current through'Ythe electronic dischargedevic'e, .which haverbeen causedby" the actionof 'themagnetic eldin passin'ghroughi the device,

Other objects and'advantages of theinventionwill` be'-v4 comje apparentfrom the'following description''taken-"`i1:iconjunction with theaccompanyingdra'wng; iriwhich Fig. 1 is a diagrammaticviewof'oneembodiment of the invention;l

Fig@ is a diagram of` arpulse'shaping"circuit suitable" for-'psc in -theinvention;

ligt is a diagrammatic view oftua mo'ditie'dform of'= the device; and

fr; 2,783,540 Ice Piatenteedl Marl 5i 135.7

Figi 4 is a diagrammatic view of" another form of the invention; I y ii.

Several forms ofxthe invention are illustrated and .will bedescribed,for` purposes of explanation; in conjuric.Y tion with avdevice Vforrneasurin'g the'lengthof va tirate-.- rial,'.such assteel,textile, paper or otherlmaterial,xwhich' has theform of a relativelylong continuous str'ipior Wire. Referring tothe :device shown in Fig.1,v a vstrip of m'a'terial 10,1whose length is to be'me'a'sured,'moves'past the lengthl measuring device, which" is stationary, and awheel 11, Whose rim is in fric'ti'onal contactv with *thev stri'p;y isrotated about itsv axis bythe'imovem'ent of the material. The .wheel ismounted? in' suitable bearings' (notlshov'vn')l on a shaft 12 supported'.in the frame'fl'of' the. length measuringdevice, .anda magnet14`,which'i n` this"` instance is of the permanent horseshoe variety,isv fixed to the wheel lfby any convenient'means, suchA as by al'str'apy15;V

Mounted? in proximity? to' the revolving' Vwheel l11,',n a position to:bela'ter described Vin detail,V is an'electronic discharge device' 16,of' the'type known. in the art asa magnetron tubeY (magneticallycontrolled diode'). Con? ventional" devices of'th'is' type' generallyconsist of-aiilaf mentary or heater type' electron-emitting-cathode centrally disposed' in Aarpgla'ss envelope'and surrounded by a concentriccylindricalanode'. In certainv types, theano'de is divided into' aplurality 'of segments,but thepmo'd'e vot operation,"as applied tothe'lpresent invention, is the same whether the anode is a' unitary cylinderorldivideddintq segments. When 'the' proper voltages are applied ton theanode' andv cathodepof ua magnetron,I electronsmtragel, radiallyfrom'the cathode to the' anode: Howeverpwhenj the magnetron is placed ina magl'leticfield, withnthe mag-` netic' lines of forcelying-,substantially parallel' tothe axis ci the cathode andy anode, thetrajectories` of the elec-Y trons are bent by the qac'tioiiof` themaghi-gtie '/eld',r As' the strength of the magnetic iieldnis increased,the trajectoriesv ofthe-electrons are'increasinglynben't, anditheelec,-` trons tend to spiral around "the cathode, with their, radial'component of velocity beingrmuch smaller thanthe angir lar component.[If thestrength of themagnetic field ,be-Y

cornes greatenoughfa cloud of electrons rotaftesfabout the filament andextends up towardnthe anodifbut few electronsfactufally reach itinThusthre is'A almost no,',cr renty dow from: anodev to cathode `underrthis' condition.k

in the` forni-of "the invention'illustrated in Fig`,.=,1, aE magnetron'"16 zrwith associated socket, V17 is 'mounted ,in a convenient'manner'isuc/li, for' example, as lon one'kend of a support'1S,'th'otheren dof' which is secured to a portion of'theffrarne' 13 ofthelengthmeasuringfdevice.v The magnetron ,16fis so positioned` withrespectto the rotatirigm'gnet 14 that, as the magnet rotates',Litszirnag neticv tie'ld'ipassesthrough the magnetron,A with. the Alinesof force substantially parallel to' the axis of thev'cath'ode and'anode'ioftlre'magnetron' i v l Voltages offthe .properV welllknownvalues aresuppliei to the' anode 4and`iilanientary, cathode orv heater,ofthe` magnetron from apulse shaping-circuit 19,latertolsev described,lin order to heat theiflame'nt and cause electrons'fto ovlvgfrom thecathode to Athe anode. A i. N Each time thatl the wheel 1,1` and. magnet1", reyolve,-y

thelines offforceof the. magnetic iieldintersect theY current fromanodefto ca'thodefof the,magnetronisI de-,v creased or interrupted, and.`the potential on theanode magnetron: 16;.' `When( this occurs, thenormal. iiow; of;

of 'Athemagnetronrises toward the..valuewdeterr nined by;

the supply voltage. Asthe magnet rotates beyond;- magnetron and the.magneticY ieldno.longer` rises' to' its normal "value 'aIid''AthfanQdefptentifal del-f 9;.. 4, .interests the magnetron, thecurrentithroughthernagnetron again.,

One of the advantages of the .invention is that the amplitudes of thecurrent pulses and anode voltage pulses are independent of the speed ofrevolution of the wheel and magnet for any reasonable speed, because thesame amount of magnetic iiux intersects themagnetron during eachrevolution of the magnet, regardless of the speed. However, the width ofthe pulses (time duration) and the slope of their leading vand trailingedges are affected bythe speed of rotation of the magnet. Therefore, itis desirable to shape the pulses in some manner such as by employingthem as triggers to actuate a circuit 19 which will provide outputpulses of uniform shape. A circuit which is suitable for this purpose isshown in Fig. 2 and is a modification of the well-known Eccles- Jordantrigger or shaping circuit.

This circuit has two stable conditions of operation in 'which one tubeis conducting and the other non-conducting, and the potential on theanode of the magnetron 16 determines which of the tubes is conducting,in accordance with well-known principles. For example, when the magneticilux from the permanent magnet 14 is not passing through the magnetronin the proper direction to reduce the current owing from the anode tothe cathode, the potential on the anode is low, because theanode-tocathode resistance of the magnetron is low. Thus, the

majority of the voltage drop in the magnetron circuit occurs acrossresistor 16a vconnected between the magnetron anode and a power supply(not shown), which also provides voltage to heat the filaments of themagnetron and the triodes 23 and 24. Since the magnetron anode isconnected to the control grid of the vacuum tube 23 through resistor1Gb, this tube will be cut ott and its anode potential will be at ornear the value of the voltage supplied from the power supply, to whichit is connected through resistor 23a. The anode of tube 23 is alsoconnected to the control grid of tube 24 through resistor 24h, whichcauses the control grid of tube 24 to be at a high potential and tube 24to be heavily conducting, with the majority of the voltage drop in itscircuit occurring across its anode resistor 24a. The potential on theanode of tube 24 is low, and, because the anode of tube 24 is connectedto the -control grid of tube 23 through resistor 23b, the potential onthe control grid of tube 23 is low. Thus, this action reinforces that ofthe magnetron to cause tube 23 to be cut 0E while tube 24 is heavilyconducting. Y

However, as the magnetic field of the magnet 14 passes through themagnetron and reduces the current ilowing therethrough, the magnetronanode and the control grid of tube 23 rise in potential, because themajority of the voltage drop in the magnetron circuit occurs across themagnetron itself. When the voltage 'at the control grid of tube 23 risesabove cut-off value, the tube 23 starts to conduct. Itsanode potentialdrops, because the voltage drop across its anode resistor 23a is largeand the voltage drop across the tube is small, which, in turn, lowersthe potential of the control grid of tube 24, because the vanode of tube23 is connected to the control grid of tube 24 through resistor 24b.This causes the anode voltage of tube 24 to rise and, because the anodeof tube 24 is connected to the control grid of tube 23 through resistor23b, the potential on the grid of tube 23 is raised still further, whichcauses tube 23 to conduct more heavily and the potential on the anode oftube 23 and on the control grid of tube 24 to be reduced further. Thus,tube 24 is quickly cut oi while tube 23 conducts heavily, and the tubesremain in this condition until the potential on the magnetron anodedecreases. Y Of course, the reverse action takes place when the magneticfield passes out of the magnetron and the magnetron current again rises.l

The output of this circuit is substantially a square wave, and may betaken conveniently from the anode of tube 24. Because the width of thesquare wave is determined by the width of the voltage pulse on themagnetron anode, it is desirable to differentiate it, and this may bedone .by a conventional circuit comprising a capacitor Z5 and a resistor26, connected in series between the anode of tube 24 and ground. Theoutput of the diterentiating circuit is taken across the resistor 26,and sharp positive and negative pulses of constant Width will appear atoutput terminals 27 in response to rises and falls of potential of theanode of tube 24. These output pulses may be amplified by -a suitableamplifier 20, before being used to trigger an electronic counter 21,which may be of the type to count either positive or negative pulses asdesired. The pulse ampliiier 20 and the electronic counter 21 arecircuits of conventional design such as are wellknown in the art, andmay be designed to operate at speeds up to several hundred kilocyclesper second.

An operator, knowing the size of the revolving wheel 1i and obtaining4from the counter 21 the number of revolutions made by the wheel, mayeasily compute the length of material which revolved the wheel. Whenmore convenient for the particular application involved, he may read thelength from va chart previously prepared for a wheel of that size, orthe wheel size and counter scale may be chosen to read directly in thedesired units of length.

Many variations and modiiications may be made in the form of theinvention shown in Fig. l. For example, the accuracy of the lengthmeasurement may be improved by counting fractions of a revolution of thewheel and magnet, which requires that there be a plurality of outputpulses from the magnetron for every revolution of the magnet. A possiblearrangement for obtaining these pulses is to mount a plurality ofmagnets on the rotating wheel and angularly space the magnets about theaxis of the wheel. Then, as the wheel rotates, the ields of the magnetsmay successively pass through the magnetron and the passing of eachmagnetic ield results in an output pulse from the magnetron. The magnetsmay be mounted on the wheel with the planes of the magnets in anyconvenient angular relationship with respect to the plane of the wheel,so long as the magnetic fields intersect the magnetron at a suitableangle as the wheel rotates. It is apparent that a similar result may beobtained by employing a plurality of magnetrons, angularly spaced aboutthe axis of the rotating wheel, and one or more magnets secured to thewheel.

Another form of the invention, which may be adapted to provide eitherone or two output pulsesfor each rotation of a wheel, is shown in Fig.3. As in the embodiment previously described, a material 29, Whoselength is to be measured, rotates a wheel 3l) as the material moves pastthe length measuring device. The wheel 30 is secured to a shaft 31,which is mounted for rotation in suitable bearings 32 and 33 insupporting portions 34 and 35 of the frame of the device, with an end31a of the shaft extending beyond the bearing 34.

A permanent horseshoe magnet 36 is secured to the end 31a of the shaftby conventional means, such as a screw 37, and a magnetron 38 with itssocket 39 is mounted adjacent the magnet 36 on a bracket 4d attached toa por tion 41 of the frame of the device. The magnetron Vis sopositioned that the magnetic field of the permanent magnet passesthrough the magnetron, and twice during each revolution of the shaft 31the lines of force of the magnetic field will be substantially parallelto the axis of the cathode and anode of the magnetron. Voltages of theproperl values may be supplied to the anode and iilamentary cathode orheater of the magnetron from a pulse shaping circuit .(not shown)similar to the circuit previously described, in order to establishcurrent ow between the anode and cathode.

The current which wouldV normally ow from the anode to the cathode ofthe magnetron 38 is'also aiected by a second permanent horseshoe magnet42 mounted adjacent the magnetron on a. support 43 removably attached tothe portion 41 of the frame of the device, and aligned with "therotatable magnet 36. The second magnet is; mountedin a fixed positionwith' the lines'v of force of.v

its magnetic field passing through the magnetron` in a directionsubstantially parallel to the axis of the magne-` tron anode andcathode, and thus it tends greatly to reduce or stop the flow of currentthrough the magnetron.

In the operation of the form of the invention shown infFig. 3, thevoltage pulses which appear on the anode of themagnetron as the wheel-39 revolves are negative rather than positive, as was the case with theembodiment shown in Fig. l. This effect is due to the combined action ofthe two permanent magnets 36 and 42. As was mentioned above, the fixedmagnet 42 tends to reduce* or stop the flow of current from anode tocathode of the magnetron, and, therefore, the potential at the magnetronanode tends to remain at or near the value of the supply voltage.However, once duringeach revoilution of the wheel 30 and magnet 36, thetwo magnets will be positioned as shown, with the north pole of oneopposite the south pole of the other, and, when this occurs, themagnetic fields of the two magnets are opposed and tend to cancel eachother. Thus, at this time, current flows from anode to cathode in themagnetron, and the potential on the anode drops. As the magnet 36rotates away from this aligned position, the current through themagnetron decreases, and the anode voltage rises. As the magnetcontinues to rotate and its field is no longer parallel to the axis ofanode and cathode of the magnetron, the magnetron currentdecreasesagain, and its anode voltage rises toward the value of thesupply voltage. When the magnet 36 has rotated 180 from the positionshown, its magnetic field reinforces that of the fixed magnet 42.However, if the fixed magnet is strong enough by itselfV to stop theflow of magnetron current, the reinforcing action of the rotating magnetwill cause no change in the voltage appearing at the magnetron anode.Thus, it is apparent that there will be one output pulse during eachrevolution of the wheel 30, and that this pulse will appear as atemporary decrease in the potential of the magnetron anode.

If it is desired to have two output pulses for each revolution of thewheel 30, the magnet 42 may be quickly removed from the device. In thiscase, the lines of force of the magnetic field of the rotatable magnet36 will pass through the magnetron in such a direction as to reduce orstop the flow of current through the magnetron twice during eachrevolution of the wheel, once when themagnet is positioned as shown, andonce when it is rotated 180 from that position. The voltage pulsesappearing on the anode of the magnetron will be positive in that theywill be temporary increases in the anode potential caused by decreasesin the current flowing from the anode to the cathode of the magnetron.

The pulse output of the magnetron may be shaped and amplified in themanner previously described, before being fed to an electronic counter.

The embodiment of the ,invention shown in Fig.I 3 may be preferred overthe form shown in Fig. l in those applications where it is desirable toconvert the `device quickly and easily to provide either one or twooutput pulses per revolution of the rotating wheel. Both forms of theinvention embody the same advantages of being frictionless, sealedagainst dirt and other contamination, and operable at any reasonablespeed.

Fig. 4 illustrates another form of the invention which may be used tocount the number of movements made by an element, such as areciprocating shaft 45. A horseshoe permanent magnet 46 is attached tothe shaft 45, and a magnetron 47 is positioned adjacent the shaft andmagnet so that, as the shaft reciprocates, the magnetic field of thepermanent magnet passes through the magnetron with the lines of forcesubstantially parallel to the axis of the magnetron anode and cathode.The magnetron 47 with its socket 4S may be mounted on one end of abracket 49, the other end of which is secured to a are. recipes-eating*shaft; 4s istantanei, battage y froma pulseshaping circuitv maybe shapesstreamlined beterey being usedtoftrigger' a counter, in the'mannerpreviously described.Vv t v v p l Itis apparent that the magnet may besol mounted: lengthwise of the shaft 45 that itsfvmagn'etic fieldpasses" through the magnetron,duringjthejejtcursion of the shaft' andagain during the return movement.'` Thus, two output1 pulses may beobtained for each cycle of' movementofft e" shaft. lf desired, themagnet may be so positioned: that its magnetic field intersects themagnetrononlyonce; that is, when the shaft has reachedthe limit vof itstravel.Y In' this case, of course, only'one output pulse would resultfrom each cycle of movementof the shaft;l p n I Although permanentmagnets havebeen illustrated and described, it is apparent thatelectromagnets might be provided to serve the same purpose as thepermanent" magnets. However, it has been found that therstrength of thefield of a. permanent magnet is generally suicient to provide outputpulses of the necessa'rymagnitude on the anode of the magnetron, and itsluseis preferredfor convenience. Y u n l i t t In some instances, it isp'ossibletopdispense withf the pulse shaping circuit and pulseamplifier,V andoperate an electromagnetic counter relay directly fromthe output of the magnetron. In this instance, the coil of the relay'would be connected in the anode circuit of the tubejand the currentowing fromv anode topcathode ofthe magnetron would also ow through therelay coil. WhenI the magnetic field of the magnet causes a decreaseinthe flow of current through the tube, the current flowingY through thecoil of the relay would also decrease and the relay be allowed either toopen or close, as appropriate for the particular arrangement employed.VThis action could be used to actuate an electromagnetic counter of anywell-known type. ,l Y

It is now apparent that the present invention provides a revolutioncounter of great utility, and one which hasy many advantages over thosepreviously known. Ot course, many changes and modifications may be madeby one skilled in the art without departing from the true spirit of theinvention, and it isinte'nded to be limited only by the scope of theappended claims. l

What isclaimed as new and desired to secure by Letters' Patent of theUnited States is:

1. A counting device, comprising an electronic dis'- charge device ofthe type wherein the flow of ycurrent' through said device may be variedbya magnetic field applied thereto, means for applying voltage to saidelecr tronic l discharge 'device' to establish` current" how there?through, at least one magnet attached to a movable element whosemovements are to be counted, said movable element being positioned toYmove said magnet past said electronic discharge device cyclicallywhereby the magnetic field of said magnet passes through said electronicdischarge device periodically as said magnet moves past the same, andmeans for indicating the flow of current through said electronicdischarge device.

2. A counting device, comprising an electronic discharge device having acathode and a concentric anode, means including connections for applyingvoltage to said electronic discharge device to establish current flowbetween said anode and cathode, at least one magnet attached to amovable element whose movements Vare to be counted, said movable elementbeing positioned to arcano move said magnet past said electronicdischarge device cyclicallyvwhereby the lines of magnetic force of saidmagnetvpass through said electronic discharge device periodically in adirection substantially parallel tothe axis of said anode and cathode assaid magnet moves past the same, thereby causing variations in thecurrent ilowbetween said anode and cathode, and means for counting saidvariations in the iiow of current between said anode and cathode.

3. A revolution counter, comprising an electronic discharge device ofthe type wherein the flow of current through said device may be variedby a magnetic held applied thereto, means for applying voltage to saidelectronic discharge device to establish current ilow therethrough, amagnet attached to a revolvable element whose revolutions are to becounted, said electronic discharge device being positioned to have themagnetic field of said magnet pass through said discharge device as saidmagnet revolves, and means for counting variations in the ow of currentthrough said electronic discharge device.

4. A revolution counter, comprising an electronic discharge devicehaving a cathode and a concentric anode, means including'connections forapplying voltage to said electronic discharge device to establishcurrent iiow between said anode and cathode, a magnet attached to arevolvable element whose revolutions are to be counted, said electronicdischarge device being positioned to have the lines of magnetic force ofsaid magnet pass through said electronic discharge device in a directionsubstantially parallel to the axis of said anode and cathode as saidmagnet revolves, thereby causing variations in the current ilow betweensaid anode and cathode, and means for counting said variations in theflow of current between said anode and cathode.

, 5. A revolution counter, comprising a magnetron, means for applyingvoltage to said magnetron to establish current flow therethrough, amagnet attached to a revolvable element whose revolutions are to becounted, said magnetron being positioned to have the lines of magneticforce of said magnet pass through said magnetron as said magnetrevolvesvand cause variations in the current flowing through saidmagnetron, and means for counting the variations in the current flowingthrough said magnetron.

6. A revolution counter, comprising a magnetron havt,

ing an anode and a cathode, means for applying voltage to said magnetronto establish current iiow between said anode and cathode, a `magnetattached to a revolvable element whose revolutions are to be counted,said magnetron being positioned to have the lines of magnetic force ofsaid magnet pass through said magnetron in a direction to causevariations in the liowof current between said anode and cathode as saidmagnet revolves thus causing voltage pulses to appear on said anode,means for shaping said Voltage pulses to provide voltage pulses f1.

of substantially constant amplitude and width, and means for countingthe shaped voltage pulses.

7. A revolution countercomprising a magnetron having a cathode and aconcentric anode, means for applying voltage to said magnetron toestablish current ow tit) between said anode and cathode, a magentmounted to be revolved by an element whose revolutions are to becounted, said magnet being positioned for its lines of magnetic force topass through said magnetron in a direc tion substantially parallel tothe axis of said anode and cathode twice during each revolution of saidelement, a second magnetV removably mounted adjacent said magnetron inposition for its lines of force to pass through said magentron in adirection substantially parallel to the axis of said anode and cathode,and means for counting variations in the ow of current between saidanode and cathode.

8. A length measuring device, comprising a wheel constructed to berevolved on its axis by movement of a material past said wheel, amagnetsecured to said wheel, a iuagentron, means for applying volt-age to saidmagnetron to establish current flow therethrough, said magnetron beingpositioned to have the lines of magnetic force oi said magnet passthrough said magnetron as said magnet revolves and cause variations inthe 4current iiowing through said magnetron, and means for counting thevariations in the current flowing through said magnetron.

9. A length measuring device, comprising a wheel constructed to berevolved on its axis by movement of a material past said wheel, a magnetsecured to said wheel, a magentron having an anode and a cathode, meansfor applying voltage to said magnetron to establish current flow betweensaid anode and cathode, said magentron being positioned to have theiines of magnetic force of said magnet pass through said magnetron in adirection to cause variations in the flow of current between said anodeand cathode and cause voltage pulses to appear on said anode as saidmagnet revolves, means for shaping said voltage pulses to providevoltage pulses of substantially constant amplitude and width, and meansfor counting the shaped voltage pulses.

A l0. A length measuring device, comprising a magnetron having a cathodeand a concentric anode, means for applying voltage to said magnetron toestablish current llow between said anode and cathode, a wheelconstructed to be revolved by movement of a material past said wheel, amagnet mounted to be revolved by said wheel, said magnet beingpositioned for its lines of magnetic force to pass through saidmagnetron in a direction substantially parallel to the axis of saidanode and cathode twice during each revolution of said wheel, a secondmagnet removably mounted adjacent said magnetron in position for itslines of magnetic force to pass through said magnetron in a directionsubstantially parallel to the axis of said anode and cathode, and meansfor counting variations in the flow of current between said anode andcathode.

References Cited in the tile of this patent UNTED STATES PATENTSl,807,098 Bodde May 26, 1931 1,947,399 Umansky Feb. 13, 1934 2,437,374Burroughs Mar. 9, 1948 2,466,251 Martin Apr. 5, 1949 2,626,536 GreenbergJan. 27, 1953 fig.

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